1. Field of the Invention
This invention relates to a method for determining an image point in an object image from an image signal made up of a series of image signal components representing picture elements in a radiation image, which has been recorded on a recording medium and which includes the object image therein.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal (image signal), and the image signal is processed and then used for reproducing the X-ray image as a visible image on a copy photograph, or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object, such as the human body. A radiation image of the object is thereby stored on the stimulable phosphor sheet. The stimulable phosphor sheet is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used during the reproduction of the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT) display device, or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials, in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light which the stimulable phosphor sheet emits when being stimulated varies over a wide range and is proportional to the amount of energy stored thereon during its exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order to obtain the desired image density, an appropriate read-out gain is set when the emitted light is being detected and converted into an electric signal to be used in the reproduction of a visible image on a recording material, such as photographic film, or on a display device, such as a CRT display device.
In order for an image signal to be detected accurately, certain factors which affect the image signal must be set in accordance with the dose of radiation delivered to the stimulable phosphor sheet and the like. Novel radiation image recording and reproducing systems which accurately detect an image signal have been proposed in, for example, U.S. Pat. No. 4,527,060. The proposed radiation image recording and reproducing systems are constituted such that a preliminary read-out operation (hereinafter simply referred to as the "preliminary readout") is carried out in order approximately to ascertain the radiation image stored on the stimulable phosphor sheet. In the preliminary readout, the stimulable phosphor sheet is scanned with a light beam having a comparatively low energy level, and a preliminary read-out image signal obtained during the preliminary readout is analyzed. Thereafter, a final read-out operation (hereinafter simply referred to as the "final readout") is carried out to obtain the image signal, which is to be used during the reproduction of a visible image. In the final readout, the stimulable phosphor sheet is scanned with a light beam having an energy level higher than the energy level of the light beam used in the preliminary readout, and the radiation image is read out with the factors affecting the image signal adjusted to appropriate values on the basis of the results of an analysis of the preliminary read-out image signal.
The term "read-out conditions" as used hereinafter means a group of various factors, which are adjustable and which affect the relationship between the amount of light emitted by the stimulable phosphor sheet during image readout and the output of a read-out means. For example, the term "read-out conditions" may refer to a read-out gain and a scale factor which define the relationship between the input to the read-out means and the output therefrom, or to the power of the stimulating rays used when the radiation image is read out.
The term "energy level of a light beam" as used herein means the level of energy of the light beam to which the stimulable phosphor sheet is exposed per unit area. In cases where the energy of the light emitted by the stimulable phosphor sheet depends on the wavelength of the irradiated light beam, i.e. the sensitivity of the stimulable phosphor sheet to the irradiated light beam depends upon the wavelength of the irradiated light beam, the term "energy level of a light beam" means the weighted energy level which is calculated by weighting the energy level of the light beam, to which the stimulable phosphor sheet is exposed per unit area, with the sensitivity of the stimulable phosphor sheet to the wavelength. In order to change the energy level of a light beam, light beams of different wavelengths may be used, the intensity of the light beam produced by a laser beam source or the like may be changed, or the intensity of the light beam may be changed by moving an ND filter or the like into and out of the optical path of the light beam. Alternatively, the diameter of the light beam may be changed in order to alter the scanning density, or the speed at which the stimulable phosphor sheet is scanned with the light beam may be changed.
Regardless of whether the preliminary readout is or is not carried out, it has also been proposed to analyze the image signal (including the preliminary read-out image signal) obtained and to adjust the image processing conditions, which are to be used when the image signal is processed, on the basis of the results of an analysis of the image signal. The term "image processing conditions" as used herein means a group of various factors, which are adjustable and set when an image signal is subjected to processing, which affect the gradation, sensitivity, or the like, of a visible image reproduced from the image signal. The proposed method is applicable to cases where an image signal is obtained from a radiation image recorded on a recording medium such as conventional X-ray film, as well as to systems using stimulable phosphor sheets.
Various methods have been proposed for calculating how the read-out conditions for the final readout and/or the image processing conditions should be adjusted on the basis of an analysis of the image signal (including the preliminary read-out image signal). As one of such methods, it has been proposed in, for example, U.S. Pat. No. 4,682,028 to create a histogram of the image signal. When a histogram of an image signal is created, the characteristics of the corresponding radiation image recorded on a recording medium, such as a stimulable phosphor sheet or X-ray film, can be ascertained based on, for example, the maximum value of the image signal, the minimum value of the image signal, or the value of the image signal at which the histogram is maximum, i.e. the value which occurs most frequently. Therefore, if the read-out conditions for the final readout, such as the read-out gain or the scale factor, and/or the image processing conditions are based on an analysis of the histogram of the image signal, it becomes possible to reproduce a visible radiation image which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness.
Also, in the course of recording a radiation image, it is often desirable for portions of the object not related to a diagnosis or the like to be prevented from being exposed to radiation. Further, when the object portions not related to a diagnosis or the like are exposed to radiation, the radiation is scattered by such portions to the portion that is related to a diagnosis or the like, and the image quality is adversely affected by the scattered radiation. Therefore, when a radiation image is recorded on the recording medium, an irradiation field stop is often used to limit the irradiation field to an area smaller than the overall recording region of the recording medium so that radiation is irradiated only to that portion of the object, which is to be viewed, and part of the recording medium.
However, in cases where the read-out conditions for the final readout and/or the image processing conditions are calculated on the basis of the results of an analysis of the image signal in the manner described above and the image signal is detected from a recording medium, on which the irradiation field was limited during the recording of the radiation image, the radiation image cannot be ascertained accurately if the image signal is analyzed without the shape and location of the irradiation field being taken into consideration. As a result, incorrect read-out conditions and/or an incorrect image processing conditions are set, and it becomes impossible to reproduce a visible radiation image which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness.
In order to eliminate the aforesaid problem, it is necessary to determine the shape and location of an irradiation field and then to calculate the read-out conditions for the final readout and/or the image processing conditions on the basis of only the image signal representing image information stored in the region inside of the irradiation field.
Accordingly, the applicant has proposed in, for example, U.S. Pat. No. 4,967,079 a novel method for accurately determining the shape and location of an irradiation field even when the irradiation field has an irregular shape. The proposed method comprises the steps of detecting a contour point, which is considered to be present on the contour of the irradiation field, on each of a plurality of radial lines each of which connects a predetermined point located in the region inside of the irradiation field with an edge of a recording medium, and determining that the region surrounded by lines connecting the thus detected contour points is the irradiation field.
In the proposed method for determining the shape and location of an irradiation field, the aforesaid predetermined point should be located in the region inside of the irradiation field, and should preferably be located in an object image, which is formed in the region inside of the irradiation field. In cases where the recording of a radiation image is carried out with an irradiation field stop, since the irradiation field stop is used to limit the irradiation field so that an image of only that portion of the object which is to be viewed is recorded, the image points in the region inside of the irradiation field (particularly, the image points at positions neighboring the center point of the irradiation field) are generally located in the region inside of the object image.
In cases where the shape and location of the irradiation field are first determined and then the image signal representing the image information recorded in the region inside of the detected irradiation field is analyzed in the manner as that described above, appropriate read-out conditions for the final readout and/or appropriate image processing conditions can be determined.
However, in order to determine the shape and location of an irradiation field with the method proposed in U.S. Pat. No. 4,967,079, it is necessary to find an image point located in the region inside of the irradiation field (preferably, an image point located in the region inside of the object image).
The image point located in the region inside of the object image should be determined when the shape and location of the irradiation field are to be recognized as described above, and when which position on a radiation image is to be employed as the center point of a visible image is determined in cases where, for example, part of the radiation image is enlarged and reproduced into the visible image.
Various methods for determining an image point in an object image have been proposed. Two examples of such methods are disclosed in U.S. patent application Ser. No. 340,744. One of the disclosed methods comprises the steps of:
i) on the basis of an image signal made up of a series of image signal components representing respective picture elements in a radiation image, which includes an object image and which has been recorded on a recording medium, finding the center of gravity on the recording medium by weighting the respective picture elements with image signal values corresponding to the respective picture elements or with the reciprocals of the image signal values, and PA1 ii) determining a position, at which the center of gravity is located, as the image point in the object image. PA1 i) on the basis of an image signal made up of a series of image signal components representing respective picture elements in a radiation image, which includes an object image and which has been recorded on a recording medium, arraying image signal values corresponding to the respective picture elements or the reciprocals of the image signal values such that the positions of the image signal values or the positions of the reciprocals of the image signal values coincide with the positions of the corresponding picture elements, PA1 ii) cumulating the image signal values or the reciprocals of the image signal values along each of two different directions on the recording medium, and plotting the resulting cumulative values of the image signal values or the resulting cumulative values of the reciprocals of the image signal values along each of the two different directions, thereby to find the distributions of the cumulative values along the two different directions, PA1 iii) detecting a coordinate point along each of the two different directions, for which point the cumulative value is approximately one half of the maximum cumulative value, from each of the distributions of the cumulative values, and PA1 iv) determining a position on the recording medium, which position is defined by the coordinate points detected along the two different directions, as the image point in the object image. PA1 i) preparing a plurality of different neural networks for a plurality of different image recording menus, each of said neural networks receiving an image signal and generating outputs which represent an image point, PA1 ii) selecting a neural network, which is optimum for said predetermined image recording menu, from the plurality of said neural networks, and PA1 iii) obtaining outputs, which represent said image point located in the region inside of said object image, from the selected neural network.
The other disclosed method comprises the steps of:
Recently, a method for utilizing a neural network has been proposed. It is considered that the method for utilizing a neural network may be applied when an image point in an object image is to be determined.
The neural network is provided with a learning function by a back propagation method. Specifically, when information (an instructor signal), which represents whether an output signal obtained when an input signal is given is or is not correct, is fed into the neural network, the weight of connections between units in the neural network (i.e. the weight of synapse connections) is corrected. By repeating the learning operation of the neural network, the probability that a correct answer will be obtained in response to a new input signal can be kept high.
By utilizing the neural network, an image point in an object image can be determined from an image signal representing a radiation image, in which the object image is included.
Specifically, an image signal representing a radiation image is fed into the neural network. From the neural network, outputs representing an image point are obtained. By repeating the learning operation of the neural network, outputs more accurately representing the image point can be obtained.
The methods disclosed in U.S. patent application Ser. No. 340,744 filed Apr. 20, 1989, now U.S. Pat. No. 5,179,597 which do not depend on an image recording menu. Specifically, with the disclosed methods, even if the image recording menu changes, determination of an image point in an object image is carried out in the same mode. However, if the image recording menu changes, the values of the image signal change. Therefore, the problem occurs in that an image point in an object image cannot be determined accurately for a certain image recording menu.
Specifically, a certain method for determining an image point in an object image is suitable only for a certain image recording menu. Therefore, for an image recording menu, a method for determining an image point in an object image, which method is optimum for the image recording menu, must be selected.
For example, when a radiation image of the head of a human body is recorded on a recording medium, a circular irradiation field stop is often used. Therefore, for a radiation image of the head of a human body, an image point must be determined, from which the shape and location of an irradiation field having one of various irregular shapes (including a circular shape) can be determined. Also, when a tomographic radiation image is recorded, the edge of an irradiation field often becomes unsharp due to flow of the image. Therefore, in such cases, an image point must be determined such that the shape and location of the irradiation field can be determined even if the edge of the irradiation field is unsharp.